Monitoring environmental noise, for instance in the vicinity of factories or mines, usually requires the installation of sound level measuring equipment. Monitoring may be required over relatively long periods of time, for instance from a few days to several months. It is usual to connect a recording device, or noise logger, to the sound level measuring equipment and to leave it unattended to generate a record of the levels of environmental noise measured by the equipment.
Where a person attends the equipment, all characteristics, including the sources and directions of noise, can be recorded by the person in addition to the levels. However, it is generally considered impractical to have a person attending noise monitoring over relatively long periods of time.
There have been limited experiments to record noise levels using a dummy human head, in an attempt to allow listeners to later judge the direction of recorded sounds. However, these experiments have been unsuccessful to date, and the proposal appears to be impractical due to the both the difficulties of leaving a dummy human head unattended, and the time required for post processing of any sound level data collected.
Sound intensity monitors indicate a resultant sound intensity taking into account all noise sources at a particular point in time. Accordingly, if two equivalent noise sources are positioned on opposite sides of a sound intensity monitor, the resultant sound intensity would be zero. While these devices do provide a form of directional indication, they have not been found suitable for environmental noise monitoring.
Similarly, a directional microphone can indicate the sound signal arising from a specific direction but generally cannot be calibrated to produce an accurate measurement of the sound level arising from that direction.
In principle, a microphone with a parabolic reflector could be calibrated to produce an accurate measurement of the sound level of noise arising from a particular direction. However, any such microphone and reflector would need to be continuously rotated in a substantially horizontal plane to detect sound from all directions. Additionally, the size of the reflector would need to be comparable with the longest wavelength to be detected, and for sound at frequencies down to 100 Hz would result in a reflector with dimensions in excess of 3 meters. Such a system has obvious practical difficulties.